Aggregates - essential to the construction
The construction industry depends on aggregates to build roads,
bridges and buildings. Different aggregates are used for different
applications. Smooth trade across European borders depends upon
standard testing methods which allow aggregate properties to be
compared easily. This project brought together 150 European laboratories
to cross test the precision of 20 aggregate tests. The project confirmed
the choice of one strength test and analysed sampling procedures.
The results are being incorporated into the relevant European standards.
Aggregates - the mass of mineral particles such as broken stones and sand - are essential to much civil engineering work. Every year in the EC, there is a demand from the construction industry for more than 2 billion tonnes of aggregates. And you only have to take a look around you to see how important aggregates are in our world's infrastructure. From tunnels and roads to buildings, bridges and power plants, they are more than just part of the scenery.
Different aggregates are right for different jobs. Smooth surfaced gravels make concrete more "workable" but crushed rock with its rough surfaces makes road surfaces skid-resistant. As the products' properties are partly determined by the properties of the aggregates, suppliers, users and testing bodies need to be able to assess the suitability of individual aggregates for different purposes.
Over the years, different countries have developed and adopted a variety of tests to meet their needs. Increasingly, these tests will be replaced by European Standard test methods. These will assist trading millions of tonnes of aggregate across European borders but only if the strengths and limitations of the new methods are fully recognised.
From 1993 to 1997, five organisations from France, Germany, The Netherlands and the UK came together to work on a programme to research and assess a range of different test methods. Between them, they coordinated a massive study involving over 150 teams of civil engineering laboratories from 16 European countries.
Cross-testing involving up to 50 laboratories took place for 20 different aggregate tests. These tests determine properties such as particle size, shape, water absorption, mechanical and weathering resistance, amongst others. One of the partners, P&S Research Ltd in the UK, analysed the results statistically and each laboratory assessed its own performance. This was the first time that this has been done on a European level.
Together, the participants discussed reasons for variations and made suggestions for improvement. As a result of this testing and analysis, precision values and comments on aggregate test methods have been incorporated in the European standards issued by the European Standardisation Committee's (CEN) Technical Committee concerned with aggregates. The project also made many laboratories more aware of the need to carry out repeatability tests and to participate in proficiency tests regularly.
The partners also confirmed the choice of a European reference strength test for aggregates by investigating existing static and dynamic tests. The team combed through the literature and unpublished data to find six candidate tests. Some of these tested wear resistance which is different to fragmentation resistance. After extensive investigation of the fragmentation resistance methods, the team compared their results. Weighing up precision, cost and potential for widespread European use lead them to choose the Los Angeles test as a reference strength test.
In the Los Angeles test, the tester places 5 kg of sample in a steel drum 70 cm in diameter with nearly 5 kg of steel balls. A dust proof lid is put on and the drum turns through 500 revolutions. At the end, the resulting fine particles are sorted using a sieve. The fraction is weighed. Clearly, the weaker aggregates will break up into smaller pieces.
Before the aggregate can be tested, there is one main difficulty: sampling. Because the aggregate consists of many slightly different sizes and types of particle, it is not easy to be sure that the tester is taking a representative sample. In fact, the errors due to sampling are, for some procedures, similar in size to the errors due to testing! The project team saw this problem and carried out 22 sampling experiments. They performed 575 sieving tests and 958 sorting tests. They recommended which are the best ways to take a bulk sample of aggregates and the number of sampling increments required.
"For me personally, one of the most important results of the project is that it differs from some standard harmonisation work where people have to defend their nation's point of view and therefore can be antagonistic. In this project, we worked together as partners and this synergism is much more efficient," continues Peter Ballmann of Bundesanstalt für Strassenwesen, one of the project partners.
"The literature study gave countries insulated by geographical position and by traditions the chance to look into the tests performed in their partner countries," says Dr Ballmann.
Without the work, it would have been more difficult to find standards that everyone agreed upon across Europe. The network of 150 laboratories will continue the good work. Some of the tests covered in this project showed poor reproducibility and there are many test methods that were not included. As each country has a national network coordinator, the newly established network will hopefully keep the process alive and perform future cross-testing experiments.
An important part of the project is communicating the results. This is being done through a web site on the Internet, a video illustrating the main aspects of the project, seminars and a 30-page summary text available in 12 European languages. It is important for industrial producers to be aware of the new standards as well. Although the partners come mainly from government organisations, they are in touch with many producers and try to take their technical needs into account.